Systems for controlling the flow of fluids, such as compressed air, natural gas, oil, propane, or the like, are generally known in the art. These systems often include at least one control valve for controlling various flow parameters of the fluid. Typical control valves include a control element such as a valve plug, for example, movably disposed within the flow path for controlling the flow of the fluid. The position of such a control element can be controlled by a positioner via a pneumatic actuator such as a piston actuator or a diaphragm-based actuator, as is known in the art. Conventional positioners deliver pneumatic signals via supply fluid to the actuator to stroke the control element of the control valve between an open and closed position, for example. The speed at which a the control valve can stroke partly depends on the size of the actuator and the flow of supply fluid contained in the pneumatic signal. For example, larger actuators/control valves typically take longer to be stroked when a positioner of equal flow output is used.
Therefore, such systems additionally employ one or more volume boosters located between the positioner and the actuator. The volume boosters are used to amplify the volume of supply fluid in relation to the pneumatic signal sent from the positioner, thereby increasing the speed at which the actuator strokes the control element of the control valve. Specifically, it should be understood by one of ordinary skill in the art that the volume booster is connected between the fluid supply and the valve actuator. Employing a pneumatic restriction in the volume booster allows large input signal changes to register on the booster input diaphragm sooner than in the actuator. A large, sudden change in the input signal causes a pressure differential to exist between the input signal and the output of the booster. When this occurs, the booster diaphragm moves to open either a supply port or an exhaust port, whichever action is required to reduce the pressure differential. The port remains open until the difference between the booster input and output pressures returns to within predetermined limits of the booster. A booster adjustment device may be set to provide for stable operation; (i.e. signals having small magnitude and rate changes pass through the volume booster and into the actuator without initiating booster operation).
However, conventional booster trim is susceptible to flow induced vibration. This vibration destabilizes the booster and often results in an audible “honking” noise being emitted from the booster. Typically this occurs at low lifts when the plug is near the seat and the vibration may occur in three-dimensional axes. This instability can happen when the booster is supplying air or when the booster is exhausting air. Such vibration or instability degrades the accuracy with which the booster can deliver a desired flow rate and causes accelerated wear of the booster trim components. This unsteady flow rate results in a variable or changing actuator velocity, which is highly undesirable.
Additionally, there are numerous applications where high capacity volume boosters are required (i.e. systems requiring volume boosters providing at least a maximum flow capacity (Cv) of seven (7.0)). Such large capacity systems may be designed with multiple volume boosters. Additionally, to maintain the large Cv, large diameter tubing is required (i.e. tubing that is at least 1″ in diameter).
Conventional volume boosters attach to the actuator via pipe components such as nipples, tees, and crosses. Control valve assemblies for large capacity systems may also use external brackets to mount the volume booster to the actuator. Such existing systems (i.e. systems that use pipe components are structural or mounting members) often require long lengths of tubing to connect the multiple volume boosters. In many applications, vibration is common. Thus, the number of boosters and the conventional connection methods make typical high flow capacity actuator assemblies susceptible to vibration induced failures resulting from the cyclic motion induced during operation. That is, large actuator applications, where multiple volume boosters and/or large Cv volume boosters are required, current state of the art mounting systems are insufficient to stabilize the volume boosters in seismically active applications (i.e. the mounting configuration is dependent on the structural integrity of the tubing and generally do not minimize the moment of the volume booster in relation to the actuator). That is, long tubing runs associated with multiple volume booster applications and conventional bracketing or mounting are very susceptible to the cyclic stresses produced by system vibration. Furthermore, in applications where high flow capacity is required traditional large diameter tubing is heavy and difficult to bend to make efficient connections leading to long tubing runs and further subjecting traditional mounting brackets to vibration induced failures as well.